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I Introduction OBJECTIVES The frontier represented by the near solar system confronts humanity with intriguing challenges and opportu- nities. With the inception of the Human Exploration and Development of Space (HEDS) enterprise in 1995, NASA has acknowledged the opportunities and has accepted the very significant challenges. This report was commissioned by NASA to assist it in coordinating the scientific information relevant to anticipating, identifying, and solving the technical problems that must be addressed throughout the HEDS program over the coming decades. Specifically, the committee was asked to ". . . undertake an assessment of scientific and related techno- logical issues facing NASA's Human Exploration and Development of Space endeavor," to ". . . look specifically at mission enabling and enhancing technologies which, for development, require an improved understanding of fluid and material behavior in a reduced gravity environment . . . [and which] might range from construction assembly techniques such as welding in space, to chemical processing of extraterrestrially derived fuels and oxygen," and to ". . . identify opportunities which exist for microgravity research to contribute to the understand- ing of fundamental science questions underlying exploration technologies and make recommendations for some areas of directed research" (see Appendix A). This report therefore sets research priorities for that portion of microgravity research that NASA may direct toward contributing to the long-term goal of HEDS technology development. In a previous report (NRC, 1995), the committee set priorities for the microgravity research that was directed primarily at increasing basic knowledge within each discipline. It should be noted that the relative balance of resources devoted to these two categories of research will be determined by NASA priorities and funding availability and that this report does not attempt to make a recommendation in that regard. In the current HEDS enterprise there are crucial technological challenges associated with travel within the solar system and with the long-term survival and productivity of missions and, ultimately, extraterrestrial colonies in environments quite different from those found on Earth. The similarities between these challenges and those faced in the sixteenth century by the explorers and colonizers who expanded the horizons of the world known to western Europe derive from what unifies all physical exploration. Participants must be able to carry out certain functions that are inevitably similar: power generation, propulsion, life support, hazards management, the ability to exploit resources encountered along the way, and so on. These functions provide us with the framework for considering the technological requirements for HEDS activities. 9
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10 MICROGRAVITY RESEARCH THE EXPLORATION ENVIRONMENT The differences between intercontinental and extraterrestrial exploration arise from the differences in how the environment affects the systems and people required for the central functions. Whereas the environmental characteristics of a space environment may be tolerated by spacecraft, they can be lethal to humans. For example, since our nominal body temperature is 37 °C and water makes up a significant part of our mass, we cannot survive in environments with ambient pressures below 62 mb, because water would begin to boil at our body temperature. It is certainly known that humans cannot survive in the vacuum of space, and that pressure limit also means that humans cannot survive on Mars by simply wearing a breathing mask. The successful Apollo missions to the Moon have demonstrated that humans can live and work on another planetary body, but for destinations beyond the Moon, it must be remembered that more time will likely be spent travelling in space during a round-trip mission than will be spent exploring the planetary body. Except for the period of acceleration during trajectory corrections, humans and their machines will spend hundreds of days in the microgravity environment around the Sun. Furthermore, the 1.28-second delay in communications signals be- tween the Moon and Earth was an irritation, but the up-to-21-minute communication delay for radio transmissions between Earth and Mars makes two-way human conversation impossible and more fundamentally means that systems being operated either in the vicinity of Mars or on its surface must be operated autonomously. The free-space environment in the inner solar system has already been characterized, along with many of the aspects of other planetary bodies that could be involved in future HEDS missions. Some of the important environmental characteristics are discussed below. Radiation in Space The National Research Council has published two studies that address the space radiation aspects of human interplanetary space travel (NRC, 1996, 1998~. Without question, it will be necessary to shield human space travelers from specific types of radiation events. Shield designs must be based on detailed knowledge of the radiation characteristics and of how the various types of radiation or particle beams interact with the shielding material and with human tissue. During low solar flare activity, the space environment is characterized by the relatively constant flux of galactic cosmic radiation. Galactic cosmic radiation can emanate from any direction and consists of approximately 87 percent protons, 12 percent helium ions, and 1 percent heavier ions, with energies ranging from 100 MeV per nucleon to 10 GeV per nucleon. In addition to galactic cosmic radiation, solar particle events associated with solar flares or solar storms can produce orders-of-magnitude increases in energetic protons. The proton fluxes can be lethal to humans who are not appropriately shielded (Parker, 1997~. Eckart (1996) has discussed the circumstances under which serious radiation hazards can be produced by major solar flare events, prolonged exposure in the Van Allen belt around Earth, and prolonged exposure to (the omnidirectional) galactic cosmic radiation outside Earth's atmospheric and magnetic shields. Research is ongoing to characterize the various hazards, to assess the cumulative effects of combinations of hazards, and to develop appropriate shielding systems. The hazards resulting from micrometeorite impacts and from collisions with space debris (in Earth orbit) should not be overlooked, but they have been studied elsewhere and do not have a major bearing on microgravity- specific research; therefore they are not covered in this report. Planetary Bodies To date, the vast majority of the design studies for human exploration missions have focused on the Moon and Mars. Missions to the Moon will probably involve longer stays than the Apollo missions and may include the development of a permanent human presence. Facilities that can support indefinite human stays will need to be much more reliable and will demand extra protection against the lunar vacuum conditions, as well as shielding against the major solar radiation events and meteorite impacts that must be anticipated for long-duration missions. For HEDS purposes a much wider range of systems must be operated for extended periods of time in the reduced
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INTRODUCTION TABLE I.1 Characteristics of Planetary Bodies in the Solar System 11 Gravity Escape Surface Surface Planetary Mean Solar Communication Level Velocity Pressure Temperature Body Distance Delay (x go) (km/s) (mbar) (K) Venus 0.723 AU 2.3-14.3 min 0.880 10.3 92,000 733 Earth (149.6 M km) (499.0 s Sun) (9.8066 m/s2) 11.2 1,013.25 288 (avg.) Moon 1.28 s 0.169 2.37 Negligible 80-390 Mars 1.524 AU 4.4-21.0 min 0.380 5.0 6-10 130-300 Phobos 0.0008-0.002 0 Negligible Deimos Variable Variable Negligible Jupiter 5.203 AU 35-52 min 2.640 61 Uncertain 124 lo 0.180 2.56 Negligible Europa 0.140 2.06 Negligible Ganymede 0.150 2.75 Negligible Callisto 0.130 2.45 Negligible Saturn 9.539 AU 1.18-1.46 h 1.150 37 Uncertain 95 Titan 0.140 2.64 1,496 94 Uranus 19.18 AU 2.52-2.80 h 1.170 22 Uncertain 58 Neptune 30.06 AU 4.02-4.31 h 1.180 25 Uncertain 56 Triton 0.081 0.31 0.02 38 SOURCE: Data from Chamberlin and Hunten (1987). gravitational environment of the Moon, where the surface gravity is 1.66 mls2 (0.169 gO) and the Sun shines continuously for almost 14 terrestrial days. Mars has an atmosphere, and its day length is almost identical to that of Earth. However, it does not have a strong enough magnetic field to shield its surface from harmful solar ionizing radiation, and it is so far from Earth that two-way interactive communication and control are impractical. Its surface gravity is 3.72 m/s2 (0.380 gO), and in many respects, Mars would be much more habitable during extended human stays than the Moon. A variety of other planetary bodies are of interest to future HEDS missions, including Jupiter's Galilean satellites, Europa and Callisto, which are now believed to contain liquid water oceans beneath their very cold water-ice shells. Titan has a surface pressure that is greater than 1 atmosphere, but its orbit around Saturn means that communications are delayed by more than 1 hour. Geysers have been observed on Triton (Soderblom et al., 1990), but it is so far from Earth that any sort of round-trip mission will require decades for completion using current technologies. Possible planetary bodies that can be considered as targets of opportunity are listed in Table I.1, along with their average distances from the Sun, average communications delay, gravitational accelerations, and surface pressure and temperature characteristics. Current knowledge of atmospheric compositions on most of those bodies has been summarized by Chamberlin and Hunten (1987~. Asteroids and comets are very different targets of opportunity because they are potential sources of raw material for space construction and for consumables, such as water and rocket propellant, and because some of these objects are potential impact threats to Earth. The satellites of Mars (Phobos and Deimos) are thought to be captured asteroids, and it is possible that some of the satellites orbiting other planets are also captured asteroids. Saturn's rings and the more tenuous rings surrounding Jupiter and Neptune are probably composed of captured asteroid and cometary debris. Excluding these captured asteroids and comets, which are less attractive sources of raw material and which pose no threat to Earth, there remain a huge number of objects that could be studied and exploited as part of a long-term HEDS program. Gradie et al. (1989) have presented an overview of the location, size, and compositional distribution of the known asteroids. The vast majority of observed asteroids have orbital semimajor axes between the orbits of Mars and Jupiter, and the main asteroid belt is considered to be between 2.1 and 3.3 AU. There are 14 classes of asteroids, and although classifications are based on combinations of albedo and spectral features (Tholen and Barucci, 1989) and do not lead directly to compositional or evolutional conclusions, they do correlate strongly
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2 MICROGRAVITY RESEARCH with orbital characteristics and size. Asteroids with diameters of 25 km or less are an order of magnitude more prevalent than asteroids with diameters of 200 km or more. Davis et al. (1989) have argued convincingly that the larger asteroids will be covered with rubble resulting from the gravitational reaccumulation of debris, after collisions, whereas the smaller asteroids lack the gravitational pull needed to recapture ejected material. Further- more, the vast majority of asteroids are rotating (tumbling) at three or four revolutions per day. Lewis et al. (1993) have examined the near-Earth asteroids (NEAs), and they estimate that there are more than 70,000 NEAs with diameters greater than 100 m. Since these objects are irregular in shape, they have variable surface gravities (assuming Phobos and Deimos are indicative of NEA irregularities, since those bodies have mean ellipsoidal radii of 13.3 x 11.1 x 9.3 km and 7.5 x 6.2 x 5.4 km, respectively, and their surface gravitational accelerations vary locally between 0.0008 go and 0.002 gal. The variety of NEA types range from ordinary chondrites, which are silicate-dominated, primitive, unmelted and undifferentiated materials (approximately 88 percent of meteorite falls) to stony irons and nearly pure iron-nickel-cobalt alloys (see Lewis and Hutson, 1993~. Comets are the most numerous objects in the solar system, with more than 10~i nuclei residing in the Oort cloud, whose aphelia are located between 20,000 and 70,000 AU from the Sun (Oort, 1990), and in all probability they represent the most primitive compositions in the solar system. Based on observations of the Giacobini-Zinner and Halley comets, it is now believed that most comets have nuclei composed of between 80 and 90 percent mixtures of (ortho- and pare-) water ice, confirming the model of Whipple (1950, 1951~. Comet nuclei are not spherical and may tend more toward prolate shapes with aspect ratios of 2: 1; however, the surveyed population is too small to permit generalization. The nuclei are known to be inhomogeneous because of the jetlike particle ejections that occur during close approach to the Sun. Huebner and McKay (1990) have discussed the high concentrations of organic molecules and their implications for the formation of life. The chemical reactions that are sustained by cometary trajectories passing near the Sun appear to explain why there is a higher concentration of organic molecules in the outer solar system than in the inner solar system. The majority of short-period comets (orbital periods of less than 200 years) that have been studied (Halley, Arend-Rigaux, Neujmin 1, Schwassmann- Wachmann 1, Tempel 2, Encke, IRAS-Araki-Alcock, and Chiron) have effective diameters on the order of 10 km, and their estimated densities are somewhat less than water ice, suggesting that they are "fluffy." Although Mars and the Moon are the focal points of current HEDS mission design studies, this study has attempted to include other targets of opportunity such as those identified in Table I.1 and certain comets and asteroids that might be selected in the future. This complementary set of planetary bodies can be explored or exploited as part of an expanded HEDS program, but more importantly it provides guidance on the range of gravitational environments that will be encountered. The influences of reduced gravity on systems that have been designed and tested on Earth to be operated on the Moon or Mars are often subtle but in many cases are predictable using straightforward scaling parameters. On the other hand, systems that are to be operated aboard manned spacecraft or on surfaces such as those of the moons of Mars, an asteroid, or a comet will be subjected to gravitational forces that are often highly variable and where steady acceleration levels are so low that the impor- tance of the basic transport processes controlling comparable terrestrial hardware designs can be either negated or overwhelmed by other phenomena. Microgravity refers to acceleration environments that are small compared with those found on Earth' s surface. This definition includes the steady gravitational environments found on the surfaces of planetary bodies such as the Moon and Mars, where the gravity level is a significant fraction of Earth's gravity, along with the highly unsteady, near-zero acceleration environments that exist on spacecraft. Both types of microgravity environment are integral to HEDS missions involving extraterrestrial bodies, but they represent very different areas of microgravity research. Furthermore, the word microgravity in the literature refers mainly to gravity levels that are less than 0.01 gO, even though the formal definition is broader. The purpose of this report is to identify and determine priorities for microgravity research, in its broader sense, that support the HEDS program. However, since it is not possible to use the broad definition of microgravity in the body of this report without generating confusion, the term fractional gravity is used to refer to gravity environments that are less than 1 gO but greater than the much lower gravitational levels found on spacecraft or on the surfaces of small planetary bodies such as Phobos and Deimos. The term microgravity is used to refer to these near-zero gravitational levels.
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INTRODUCTION 13 REPORT ORGANIZATION AND DEVELOPMENT The overriding challenge posed by the charge to the committee is twofold. Two diverse groups of objects- the engineering subsystems necessary for HEDS functions and the microgravity-specific physical phenomena that affect them must first be identified. Then, dependencies between the two groups must be adduced, and questions derived about the fundamental scientific obstacles to the enabling technologies. Once these questions have been derived, the task boils down to one of enumerating as many as possible of the known (and possibly unknown) problem areas in which further scientific inquiry can be expected to produce solutions. Previous NRC reports have dealt with exploiting microgravity as a parameter in pursuit of new basic science (NRC, 1992, 1995~. Priorities were assigned to microgravity research topics primarily on the basis of their potential to expand basic knowledge within a given discipline. To meet the goals of this current study an additional step was needed: the committee first examined the technological barriers and only then asked what scientific research would level them. The organization of this report reflects this approach (Figure I. 1~. The committee first lists in some detail the technological requirements for HEDS (Chapter III), coming to the underlying scientific phenomena only after assembling this list. Chapters III and IV constitute the heart of the report. They are essentially parallel construc- tions: the first (Chapter III) identifies the HEDS technologies and describes their microgravity-dependent sub- systems, and the second (Chapter IV) identifies the physical bases for microgravity sensitivity and describes the associated phenomena. A logical symmetry between Chapters III and IV arises from the links between them: microgravity sensitivities generate potential failure modes in the subsystems; the need to improve subsystem efficiency drives the generation of relevant new scientific questions associated with microgravity-induced sensi- tivity. Several important issues that lie outside the purview of this parallelism are developed in Chapter V. Finally, having derived the scientific questions in this manner, the committee makes research recommendations in Chapter VI and programmatic recommendations in Chapter VII. FIGURE I. 1 Structured map of the logical dependen- cies between material presented in different chapters of this report.
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14 MICROGRAVITY RESEARCH In developing the material for this report the committee first considered the broad capabilities that the HEDS program must have in order to succeed. These were judged to be the following: · Power generation and storage, · Space propulsion, · Life support, · Hazard control, · Material production and storage, and · Construction and maintenance. Certain important functions, such as communications, were excluded because gravity plays no significant role. Within each of the above functions, various enabling technologies were considered, and these are discussed in Chapter III. Some of the technologies were highly speculative since the committee did not rule out NASA's far- term needs from its considerations. These technology systems were then broken down into subsystems (or in some cases processes), which is the level at which most m~crogravity effects are expected to intervene. This last was a critical step because it was impractical for the committee to consider in detail all of the systems that have been suggested for use in HEDS programs or to predict the new systems that might be developed in the next few decades. However, many basic subsystems, such as radiators, show up repeatedly in a wide range of technologies, and their use is likely to continue in future technologies. Therefore, by examining a diverse, but not comprehen- sive, set of systems, the committee was able to identify the most commonly occurring subsystems and consider how their performance might be affected by reduced gravity. Of the various phenomena known to become problematic in m~crogravity and considered in Chapter IV, one phenomenon underlies nearly all the issues the committee identified as problem areas worthy of research invest- ment. This phenomenon is the profound effect of gravitation on the separation of distinct phases, particularly fluid phases. Terrestrial gravity fields provide reliable separations based on density differences; in the near absence of gravitational forces, phases of different density do not spontaneously separate. The lack of phase separation in microgravity has severely compromised a range of promising technologies associated with all HEDS functions, from propulsion to sanitation. An inability to reliably predict multiphase behavior in microgravity has led NASA to utilize single-phase systems even though functioning two-phase systems would have substantially enhanced efficiency. Chapter V addresses the indirect effects of m~crogravity on system design, various ways to counter m~cro- gravity, and also certain program issues such as experimental design matrices and the development of physics- based predictive models and probabilistic risk assessment. The concerns developed in Chapters III through V are integrated into a discussion of research recommenda- tions and priorities in Chapter VI. There, the committee assesses how the technical problems described in Chapter III can best be solved by addressing the underlying physical phenomena outlined in Chapter IV. Finally, in Chapter VII, important programmatic issues are addressed that are critical to the conduct of the recommended research. REFERENCES Chamberlin, J.W., and D.M. Hunten. 1987. Theory of Planetary Atmospheres, 2nd Ed. New York: Academic Press. Davis, D.R., S.J. Weidenschilling, P. Farinella, P. Paolicchi, and R.P. Binzel. 1989. Asteroid collisional history: Effects on sizes and spins. Pp. 805-826 in Asteroids II. R.P. Binzel, T. Gehrels, and M.S. Matthews, eds. Tucson: University of Arizona Press. Eckart, P. 1996. Spaceflight Life Support and Biospherics. Torrance, Calif.: Microcosm Press, and Dordrecht, Netherlands: Kluwer Academic Publishers. Gradie, J.C., C.R. Chapman, and E.F. Tedesco. 1989. Distribution of taxonomic classes and the compositional structure of the asteroid belt. Pp. 316-335 in Asteroids II. R.P. Binzel, T. Gehrels, and M.S. Matthews, eds. Tucson: University of Arizona Press. Huebner, W.F., and C.P. McKay. 1990. Implications of comet research. Pp. 305-331 in Physics and Chemistry of Comets. New York: Springer-Verlag.
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INTRODUCTION 15 Lewis, J.S., and M.L. Hutson. 1993. Asteroidal resource opportunities suggested by meteorite data. Pp. 523-542 in Resources of Near-Earth Space. J. Lewis, M.S. Matthews, and M.L. Guerrieri, eds. Tucson and London: University of Arizona Press. Lewis, J.S., M.S. Matthews, and M.L. Guerrieri, eds. 1993. Resources of Near-Earth Space. Tucson and London: University of Arizona Press. National Research Council (NRC), Space Studies Board (SSB). 1992. Toward a Microgravity Research Strategy. Washington, D.C.: National Academy Press. NRC, SSB. 1995. Microgravity Research Opportunities for the 1990s. Washington, D.C.: National Academy Press. NRC, SSB. 1996. Radiation Hazards to Crews of Interplanetary Missions: Biological Issues and Research Strategies. Washington, D.C.: National Academy Press. NRC, SSB. 1998. A Strategy for Research in Space Biology and Medicine in the New Century. Washington, D.C.: National Academy Press. Oort, J.H. 1990. Orbital distribution of comets. Pp. 235-244 in Physics and Chemistry of Comets. New York: Springer-Verlag. Parker, E.N. 1997. Mass ejection and a brief history of the solar wind concept. Pp. 3-27 in Cosmic Winds and the Heliosphere. J.R. Jokipii, C.P. Sonett, and M.S. Giampapa, eds. Tucson: University of Arizona Press. Soderblom, L.A., S.W. Kieffer, T.L. Becker, R.H. Brown, A.F. Cook II, C.J. Hansen, T.V. Johnson, R.L. Kirk, and E.M. Shoemaker. 1990. Triton's geyser-like plumes: Discovery and basic characterization. Science 250:410-415. Tholen, D.J., and M.A. Barucci. 1989. Asteroid taxonomy. Pp. 298-315 in Asteroids II. R.P. Binzel, T. Gehrels, and M.S. Matthews, eds. Tucson: University of Arizona Press. Whipple, F.L. 1950. A comet model I: The acceleration of comet Encke. Astrophys. J. 111:375-394. Whipple, F.L. 1951. A comet model II: Physical relations for comets and meteors. Astrophys. J. 113:464-474.
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Representative terms from entire chapter: